U.S. patent number 7,565,045 [Application Number 12/107,906] was granted by the patent office on 2009-07-21 for tunable light source apparatus, and adjustment method and control program of the same.
This patent grant is currently assigned to NEC Corporation. Invention is credited to Kouichi Suzuki.
United States Patent |
7,565,045 |
Suzuki |
July 21, 2009 |
Tunable light source apparatus, and adjustment method and control
program of the same
Abstract
The present invention aims to provide a tuning light source
apparatus including a multiple-optical resonator, where the
resonance frequencies of the respective optical resonators in a
multiple-optical resonator are exactly coincided with the set
frequency, and the frequency of the output laser beam is locked
within a range of about 1 GHz from the set frequency. Current is
flowed to TO phase shifters based on lights detected by light
receiving elements, and the resonance wavelengths of resonators are
adjusted in an aim of obtaining a state in which an intensity of an
oscillation light becomes a maximum and at the same time an
intensity of a light from a through port becomes a minimum.
Inventors: |
Suzuki; Kouichi (Tokyo,
JP) |
Assignee: |
NEC Corporation (Tokyo,
JP)
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Family
ID: |
39522414 |
Appl.
No.: |
12/107,906 |
Filed: |
April 23, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080259437 A1 |
Oct 23, 2008 |
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Current U.S.
Class: |
385/27; 385/30;
385/15; 359/326 |
Current CPC
Class: |
H01S
5/0687 (20130101); H01S 5/141 (20130101); H01S
5/1032 (20130101); H01S 5/065 (20130101); H01S
5/142 (20130101); H01S 5/028 (20130101); H01S
5/0287 (20130101); H01S 5/0656 (20130101); H01S
5/06246 (20130101) |
Current International
Class: |
G02B
6/26 (20060101) |
Field of
Search: |
;385/27-30,14,15,24
;359/326 ;372/20 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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62100706 |
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May 1987 |
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JP |
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2003527625 |
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Sep 2003 |
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JP |
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2005327881 |
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Nov 2005 |
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JP |
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2006196554 |
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Jul 2006 |
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JP |
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Primary Examiner: Kim; Ellen
Claims
What is claimed is:
1. A tuning light source apparatus comprising: an optical resonator
filter including a multiple-optical resonator in which a plurality
of optical resonators of different light path lengths is connected,
the optical resonators comprising: a first optical resonator
positioned at a light input/output side of the optical resonator
filter; a second optical resonator positioned at a reflection side
of the optical resonator filter; a third optical resonator
positioned between the first and the second optical resonators; a
light supply device for supplying light to the optical resonator
filter at the light input/output side of the optical resonator
filter; a tuning device for changing a resonance wavelength of the
multiple-optical resonator; a first light detection device for
detecting an oscillation light output to outside from an output
port of the optical resonator filter, the output port located at
the light input/output side of the optical resonator filter; a
plurality of additional light detection devices for detecting light
deviated from corresponding resonator paths of the optical
resonator filter output from corresponding through ports of the
optical resonator filter, the additional light detection devices
comprising: a first additional light detection device to detect
light deviated from a first resonator path of the optical resonator
filter output from a first through port of the optical resonator
filter, the first resonator path located at the light input/output
side of the optical resonator filter, the first additional light
detection device to detect the light output by the light supply
device through the first resonator path that does not reach the
first, the second, and the third optical resonators; a second
additional light detection device to detect light deviated from a
second resonator path of the optical resonator filter output from a
second through port of the optical resonator filter, the second
resonator path located between the first and the second optical
resonators, the second additional light detection device to detect
the light output by the light supply device through the first and
the second resonator paths and the first optical resonator that
does not reach the second and the third optical resonators; a third
additional light detection device to detect light deviated from a
third resonator path of the optical resonator filter output from a
third through port of the optical resonator filter, the third
resonator path located at the reflection side of the optical
resonator filter, the third additional light detection device to
detect the light output by the light supply device through the
first, the second, and the third resonator paths, and the first and
the third optical resonators, that does not reach the second
optical resonator; and a control unit for controlling the operation
of the tuning device based on the lights detected by the first and
the additional light detection devices.
2. The tuning light source apparatus according to claim 1, wherein
the control unit has a function of controlling the operation of the
tuning device so that an intensity of the light detected by the
first light detection device takes a maximum value and, at the same
time, an intensity of the light detected by one or more of the
additional light detection devices takes a minimum value.
3. The tuning light source apparatus according to claim 1, wherein
each additional light detection device corresponds to one of the
optical resonators.
4. The tuning light source apparatus according to claim 1, wherein
the tuning device is configured to individually change a resonance
wavelength of each optical resonator in the multiple-optical
resonator.
5. The tuning light source apparatus according to claim 1, wherein
each optical resonator in the multiple-optical resonator is a ring
resonator.
6. The tuning light source apparatus according to claim 1, wherein
the tuning device is a film shaped heater for changing a light path
length of each optical resonator.
7. The tuning light source apparatus according to claim 1, wherein
the light supply device is a semiconductor optical amplifier.
8. The tuning light source apparatus according to claim 1, wherein
the optical resonator filter is a planar lightwave circuit.
9. A tuning light source apparatus comprising: an optical resonator
filter including a multiple-optical resonator in which a plurality
of optical resonators of different light path lengths is connected,
the optical resonators comprising: a first optical resonator
positioned at a light input/output side of the optical resonator
filter; a second optical resonator positioned at a reflection side
of the optical resonator filter; a third optical resonator
positioned between the first and the second optical resonators; a
light supply means for supplying light to the optical resonator
filter at the light input/output side of the optical resonator
filter; a tuning means for changing a resonance wavelength of the
multiple-optical resonator; a first light detection means for
detecting an oscillation light output to outside from an output
port of the optical resonator filter; a plurality of additional
light detection means for detecting light deviated from
corresponding resonator paths of the optical resonator filter
output from corresponding through ports of the optical resonator
filter, the additional light detection means comprising: first
additional light detection means for detecting light deviated from
a first resonator path of the optical resonator filter output from
a first through port of the optical resonator filter, the first
resonator path located at the light input/output side of the
optical resonator filter, the first additional light detection
means to detect the light output by the light supply device through
the first resonator path that does not reach the first, the second,
and the third optical resonators; second additional light detection
means for detecting light deviated from a second resonator path of
the optical resonator filter output from a second through port of
the optical resonator filter, the second resonator path located
between the first and the second optical resonators, the second
additional light detection means to detect the light output by the
light supply device through the first and the second resonator
paths and the first optical resonator that does not reach the
second and the third optical resonators; third additional light
detection means for detecting light deviated from a third resonator
path of the optical resonator filter output from a third through
port of the optical resonator filter, the third resonator path
located at the reflection side of the optical resonator filter, the
third additional light detection means to detect the light output
by the light supply device through the first, the second, and the
third resonator paths, and the first and the third optical
resonators, that does not reach the second optical resonator; and a
control means for controlling the operation of the tuning means
based on the lights detected by the first and the additional light
detection means.
10. A light source wavelength adjustment method comprising:
supplying light into an optical resonator filter from a light
supply device at a light input/output side of the optical resonator
filter, the optical resonator filter having: a first optical
resonator positioned at the light input/output side of the optical
resonator filter; a second optical resonator positioned at a
reflection side of the optical resonator filter; a third optical
resonator positioned between the first and the second optical
resonators; detecting an oscillation light output outside from an
output port of the optical resonator filter using a first light
detection device; detecting light deviated from a plurality of
resonator paths in the optical resonator filter from a plurality of
corresponding through ports of the optical resonator filter using a
first additional light detection device, a second additional light
detection device, and a third additional light detection device,
where: the first additional light detection device is to detect
light deviated from a first resonator path of the optical resonator
filter output from a first through port of the optical resonator
filter, the first resonator path located at the light input/output
side of the optical resonator filter, the first additional light
detection device to detect the light output by the light supply
device through the first resonator path that does not reach the
first, the second, and the third optical resonators, the second
additional light detection device is to detect light deviated from
a second resonator path of the optical resonator filter output from
a second through port of the optical resonator filter, the second
resonator path located between the first and the second optical
resonators, the second additional light detection device to detect
the light output by the light supply device through the first and
the second resonator paths and the first optical resonator that
does not reach the second and the third optical resonators, the
third additional light detection device is to detect light deviated
from a third resonator path of the optical resonator filter output
from a third through port of the optical resonator filter, the
third resonator path located at the reflection side of the optical
resonator filter, the third additional light detection device to
detect the light output by the light supply device through the
first, the second, and the third resonator paths, and the first and
the third optical resonators, that does not reach the second
optical resonator; and controlling the operation of a tuning device
based on each detected light.
11. The light source wavelength adjustment method according to
claim 10, further comprising controlling the operation of the
tuning device so that an intensity of the oscillation light from
the output port to the outside takes a maximum value and, at the
same time, an intensity of the light from the through ports takes a
minimum value.
12. A light source wavelength adjustment program for causing a
computer for controlling the operation of a tuning light source
apparatus including an optical resonator filter with a
multiple-optical resonator in which a plurality of optical
resonators is connected, a light supply device for supplying light
into the filter from an input port of the optical resonator filter,
and a tuning device for changing a resonance wavelength of the
multiple-optical resonator to execute functions of: instructing the
light supply device to supply light into the optical resonator
filter at a light input/output side of the optical resonator
filter, the optical resonator filter having: a first optical
resonator positioned at the light input/output side of the optical
resonator filter; a second optical resonator positioned at a
reflection side of the optical resonator filter; a third optical
resonator positioned between the first and the second optical
resonators; inputting intensity data of a light output to the
outside from an output port of the optical resonator filter from a
light detection device arranged in advance, via a first light
detection device; inputting intensity data of light deviated from a
plurality of resonators path in the optical resonator filter output
from corresponding through ports of the optical resonator filter
from a light detection device arranged in advance, via a first
additional light detection device, a second additional light
detection device, and a third additional light detection device,
where: the first additional light detection device is to detect
light deviated from a first resonator path of the optical resonator
filter output from a first through port of the optical resonator
filter, the first resonator path located at the light input/output
side of the optical resonator filter, the first additional light
detection device to detect the light output by the light supply
device through the first resonator path that does not reach the
first, the second, and the third optical resonators, the second
additional light detection device is to detect light deviated from
a second resonator path of the optical resonator filter output from
a second through port of the optical resonator filter, the second
resonator path located between the first and the second optical
resonators, the second additional light detection device to detect
the light output by the light supply device through the first and
the second resonator paths and the first optical resonator that
does not reach the second and the third optical resonators, the
third additional light detection device is to detect light deviated
from a third resonator path of the optical resonator filter output
from a third through port of the optical resonator filter, the
third resonator path located at the reflection side of the optical
resonator filter, the third additional light detection device to
detect the light output by the light supply device through the
first, the second, and the third resonator paths, and the first and
the third optical resonators, that does not reach the second
optical resonator; and controlling the operation of the tuning
device based on each intensity data input in a through light
intensity input process and an output light intensity input
process.
13. The light source wavelength adjustment program according to
claim 12, wherein content is specified to control the operation of
the tuning device so that an intensity of the light output from the
output port to the outside takes a maximum value and, at the same
time, an intensity of the light output from the through ports takes
a minimum value in the tuning process.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority
from Japanese patent application No. 2007-112642, filed on Apr. 23,
2007, the disclosure of which is incorporated herein in its
entirety by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to tunable light source apparatuses
used in optical fiber communication, light source wavelength
adjustment methods, and light source wavelength adjustment
programs, in particular, to a tunable light source apparatus used
in an optical communication of WDM (Wavelength Division
Multiplexing) transmission system, a light source wavelength
adjustment method, and a light source wavelength adjustment
program.
2. Related Art
Recently, in the field of optical communication, the WDM
transmission system of converting a plurality of data signals to
light signals of different wavelengths, and multiplexing the
plurality of light signals of different wavelengths to be
transmitted in one optical fiber thereby realizing a large capacity
optical transmission is widely put to practical use. Further,
practical application of the DWDM (Dense Wavelength Division
Multiplexing) transmission system, which can realize denser
multiplexing than the WDM technique, is advancing.
In the optical communication system of the WDM transmission system,
the transmission light wavelength is set on a frequency grid
(ITU-Grid) standardized by the ITU (International Telecommunication
Union). Thus, for every wavelength on the ITU-Grid, a corresponding
light source is required respectively, and furthermore, since the
ITU-Grid frequency spacing is set to be small in the DWDM, the
number of settable wavelength increases, and greater number of
light sources are required. In order to solve such disadvantage,
practical application of a tunable light source apparatus in which
the output wavelength can be freely controlled is advancing.
In order to realize a reliable optical communication, the tunable
light source apparatus needs to set the frequency of the output
light on the ITU-Grid, and continuously lock the frequency of the
output light within a frequency range of about 1 GHz from such set
frequency. A multiple-optical resonator type tunable light source
apparatus serving as the tunable light source apparatus described
above is disclosed in Japanese Laid-Open Patent Publication No.
2006-196554 (Patent Document 1).
The tunable light source apparatus of Patent Document 1 has a
configuration in which laser oscillation occurs at a wavelength at
which all three transmission resonance frequencies of three ring
resonators coincide, where the desired laser oscillation frequency
is output by adjusting the input power to a TO (Thermo-Optic) phase
shifter arranged in each ring resonator.
In such tunable light source apparatus, the filter loss becomes the
smallest, and the stability of the wavelength and the power
tolerance of the TO can be maximized when the resonance frequencies
of the three ring resonators are exactly coinciding on the
oscillation frequency.
However, in the multi-purpose tunable light source apparatus, the
laser oscillation wavelength fluctuates and deviates from the
ITU-Grid due to change in outside temperature and variation in
refractive index of the light waveguide portion. Particularly,
since the tunable light source apparatus of external resonator
type, such as PLC (Planar Lightwave Circuit) type, has a structure
in which the oscillation frequency can be freely changed, and thus
has a characteristic in that the wavelength tends to easily vary
inherently. For this reason, in order to maintain the stability of
the laser wavelength over a long period of time, the shift of
central wavelength needs to be detected and corrected with various
methods.
In the tunable light source disclosed in patent document 1,
frequency ripples having a period of various sizes are generated
due to the presence of various minor reflections such as internal
reflection of an SOA (Semiconductor Optical Amplifier), PLC/SOA
connecting point reflection and the like. Since the frequency and
the intensity of the ripple are changed depending on gain current,
environment temperature, and the like, the condition in which the
exit light level and the SMSR (Sub-Mode Suppression Ratio) become a
maximum also changes.
Due to the influence of such ripples, a state in which the central
frequencies in the three ring resonators do not coincide arises
even if the exit light level is a maximum, and whether or not the
three central frequencies are exactly coinciding on the oscillation
frequency is difficult to be determined only from the
characteristics of the output exit laser beam.
In the multiple-optical resonator disclosed in patent document 1,
when laser oscillation starts at a certain wavelength, the gain is
concentrated thereby other sub-oscillation modes are suppressed,
and thus laser oscillation similarly occurs even if the central
frequencies in the three ring resonators are slightly shifted, and
whether or not the three central frequencies are accurately
coinciding on the oscillation frequency is difficult to be
determined only from the characteristics of the exit light.
A mode gain difference, which is the transmission loss difference
between one oscillating wavelength channel and an adjacent
oscillating channel, in a spectrum of a single mode laser from the
PLC multiple-optical resonator disclosed in patent document 1 takes
a maximum value when all three central frequencies in the three
ring resonators exactly coincide. Disadvantages such as unexpected
wavelength skipping etc. arise unless the assumed mode gain
difference is obtained. If the wavelength of the tunable light
source automatically switches to another wavelength due to
wavelength skipping, the communication of the relevant wavelength
becomes disconnected, and furthermore, the communication of another
wavelength channel also becomes disconnected.
In the multiple-optical resonator type tunable light source
apparatus of Patent Document 1, in order to increase the mode gain
difference, a method of increasing the finesse of the optical
resonator filter may be adopted. The frequency characteristics of
the optical resonator filter becomes of narrower band and the gain
difference also becomes larger as the number of average turns of
the ring resonator is increased. If the average number of turns
increases, the propagation loss accumulates and the insertion loss
increases as it passes through the ring resonator that many times,
and thus a trade off state such that the output of the light source
becomes difficult to be obtained arises. Thus, the design is
required to be made at a necessity minimum finesse, and it is
important to reliably coincide the central frequencies of the three
ring resonators in the multiple-optical resonator to obtain an
optimum magnitude for the mode gain difference.
SUMMARY OF THE INVENTION
It is an exemplary object of the invention to provide a tunable
light source apparatus including a multiple-optical resonator,
where the central frequencies of the respective optical resonators
in the multiple-optical resonator are exactly coincided with the
set frequency to obtain an optimum magnitude for the mode gain
difference, and the frequency of the output laser light is locked
within a range of about 1 GHz from the set frequency.
In order to achieve the above object, a tunable light source
apparatus according to an exemplary aspect of the invention relates
to a tunable light source apparatus including an optical resonator
filter including a multiple-optical resonator in which a plurality
of optical resonators with different light path lengths is
connected; a light supply device for supplying light to the optical
resonator filter; a tunable device for changing a resonance
wavelength of the multiple-optical resonator; a first light
detection device for detecting an oscillation light output to
outside from an output port of the optical resonator filter; a
second light detection device for detecting a light deviated from a
resonator path of the optical resonator filter output from a
through port of the optical resonator filter; and a control unit
for controlling the operation of the tunable device based on the
lights detected by the first and the second light detection
device.
A wavelength variable light source apparatus control method
according to another exemplary aspect of the invention relates to a
light source wavelength adjustment method including the steps of
supplying light into an optical resonator filter from a light
supply device; detecting an oscillation light output outside from
an output port of the optical resonator filter and detecting a
light deviated from a resonator path in the optical resonator
filter from a through port of the optical resonator filter; and
controlling the operation of a tunable device based on each
detected light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view showing a configuration of a tunable light
source apparatus according to one exemplary embodiment of the
invention;
FIG. 2 is a view showing wavelength response characteristics of a
multiple-optical resonator according to the exemplary embodiment
described in FIG. 1;
FIG. 3 is a block diagrams showing a configuration of a control
unit according to the exemplary embodiment described in FIG. 1;
FIGS. 4A and 4B are views showing examples of SOA phase
characteristics of an output light level detected by the light
receiving elements in the exemplary embodiment described in FIG.
1;
FIG. 5 is a flowchart showing operation of the control unit
according to the exemplary embodiment described in FIG. 1;
FIG. 6 is a view showing TO tolerance of a light receiving element
for detecting light that did not enter a coarse tuning ring
resonator according to the exemplary embodiment described in FIG.
1;
FIG. 7 is a view showing TO tolerance of a light receiving element
for detecting light that did not enter a fine tuning ring resonator
according to the exemplary embodiment described in FIG. 1;
FIG. 8 is a view showing TO tolerance of a light receiving element
for detecting light that did not enter an ITU-Grid fixing ring
resonator according to the exemplary embodiment described in FIG.
1;
FIG. 9 is a view showing TO tolerance of an output wavelength of
the oscillation light in the exemplary embodiment described in FIG.
1;
FIG. 10 is a view showing TO tolerance of a light receiving element
for detecting the oscillation light in the exemplary embodiment
described in FIG. 1; and
FIG. 11 is a plan view showing another example of a configuration
of the tunable light source apparatus according to the exemplary
embodiment described in FIG. 1.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
One exemplary embodiment of the present invention will now be
described with reference to the drawings.
FIG. 1 is a plan view showing a tunable light source apparatus 10
of the exemplary embodiment.
As shown in FIG. 1, the tunable light source apparatus 10 of the
exemplary embodiment includes an optical resonator filter 11, or a
PLC (Planar Lightwave Circuit), configured by forming a
multiple-optical resonator 20 in which ring resonators 21, 22, and
23 with different light path lengths are connected via waveguides
24, 25, an input/output side optical waveguide 26 on the side for
inputting/outputting the laser light, and a reflection side optical
waveguide 27 including a light reflection film 12 arranged on one
end; and an SOA (Semiconductor Optical Amplifier) 13 serving as a
light supply device connected to a light input/output port 26i of
the optical resonator filter 11.
Furthermore, the tunable light source apparatus 10 of the exemplary
embodiment is configured including TO phase shifters 14 and 15
serving as tunable device for changing the resonance wavelength in
the multiple-optical resonator 20; a prism coupler 16 for
reflecting the light quantity of about one tenth of the incident
light in the direction which is turned by 90 degrees; a light
receiving element 18 serving as a first light detection device for
detecting the oscillation light resonated by the multiple-optical
resonator 20 and output to the outside from the input/output port
26i; and light receiving elements 19a, 19b, and 19c serving as
second light detection device for detecting light from through
ports 26t, 24t, and 25t deviated from the resonator path in the
optical resonator filter 11; and further includes a control unit 17
for controlling application powers to the TO phase shifters 14 and
15 based on the light detected by the light receiving elements 18,
19a, 19b, and 19c.
As shown in FIG. 1, the optical resonator filter 11 is a PLC
substrate configured so that the multiple-optical resonator 20 is
arranged between the input/output side waveguide 26 and the
reflection side waveguide 27. The ring resonators 21, 22, and 23
and the light waveguides 24, 25, 26, and 27 in the PLC substrate
are formed with quartz glass waveguide etc. in which quartz glass
is deposited on a silicon substrate or glass substrate.
The multiple-optical resonator 20 is formed by connecting in series
the ring resonators 21, 22, and 23 in which the light path length
represented by the product of the refractive index of a medium
through which the light propagates and the geometrical length
differs from each other. The multiple-optical resonator 20 combines
and splits the light of resonance wavelength only when the ring
resonators 21, 22, and 23 are simultaneously resonating, so that a
large FSR (Free Spectral Range) is obtained by a Vernier
effect.
Vernier effect is a phenomenon where when a plurality of resonators
having different light path lengths is combined, the resonance
frequency of each resonator in which the peak period is shifted
overlaps at the frequency of least common multiple thereof. The
multiple-optical resonator in which the plurality of resonators are
combined utilizes the Vernier effect so that the apparent FSR
becomes the frequency of least common multiple of the resonance
frequencies of the respective resonators. The characteristics of
the frequency then can be controlled in a wider range than with a
single resonator.
The structure of the tunable light source apparatus 10 uses the
wavelength transmission characteristics of each drop port of the
ring resonators 21, 22, and 23 to select the resonance mode and
perform the single mode oscillation. According to the design of
slightly differing the light path lengths of the ring resonators
21, 22, and 23 of three stages configuring the multiple-optical
resonator 20, the respective resonance wavelengths coincide at only
one location even in a wide wavelength range of a few dozen nm, and
the single mode oscillation occurs at such coinciding
wavelength.
In the optical resonator filter 11 according to the exemplary
embodiment, the resonance wavelength in the multiple-optical
resonator 20, which is the wavelength of least common multiple of
the respective resonance wavelengths of the ring resonators 21, 22,
and 23, is set as the wavelength on the ITU-Grid by fixing the FSR
of the ring resonator 21 on the ITU-Grid. In the multiple-optical
resonator 20, the ring resonator 21 is provided for ITU-Grid
fixation, the ring resonator 22 is provided for fine tuning, and
the ring resonator 23 is provided for coarse tuning.
The TO phase shifters 14 and 15 shown in FIG. 1 are mounted on the
optical resonator filter 11 in correspondence to the position of
the ring shaped waveguide of the ring resonators 22 and 23. The
refractive index of the ring shaped waveguides of the ring
resonators 22 and 23 using glass or compound semiconductor changes
according to temperature. Therefore, the TO shift shifters 14 and
15 individually changes the refractive index of the waveguide
through application of heat to the ring shaped waveguides of the
ring resonators 22 and 23 to change the respective light path
lengths of the ring resonators 22 and 23 and change the resonance
wavelength in the multiple-optical resonator 20.
In the tunable light source apparatus 10 of the exemplary
embodiment, a film heater made of aluminum film, which is vapor
deposited on a position corresponding to the ring resonators 22 and
23 of the optical resonator filter 11, may be arranged as the TO
phase shifters 14 and 15. The light path lengths of the ring
resonators 22 and 23 are controlled with thermooptical effect by
the TO phase shifters 14 and 15. The resonance wavelength in the
multiple-optical resonator 20 can be changed by variable
controlling the light path lengths of the ring resonators 22 and 23
simultaneously.
FIG. 2 is a view showing wavelength response characteristics of the
multiple-optical resonator 20 seen from the SOA 13 side. In this
case, the TO phase shifter 14 is provided as a tunable device for
coarse tuning, and the TO phase shifter 15 is provided as a tunable
device for fine tuning.
The SOA 13 has the end face on the optical resonator filter 11
side, which is performed with AR coating, and the external output
side end face, opposite thereto, a reflectivity of which is set to
be higher than the AR coated end face on the opposite side by about
1 to 20%. The SOA 13 includes a phase control region 13A, where the
wavelength of the light exit from the SOA 13 is changed by
controlling the phase current carried to the phase control region
13A. Thus, adjustment is made so that the phases at the surface on
the optical resonator filter 11 side of the SOA 13 and the high
reflection film 12 coincide, and the insertion loss of the filter
can be suppressed. In the exemplary embodiment, the SOA 13 is used
as the light supply device, but is not limited thereto, and an
optical amplifier such as optical fiber amplifier, light source
such as semiconductor laser, and the like may be used.
The light receiving elements 18, 19a, 19b, and 19c are photodiodes.
The light receiving element 18 is arranged at a position of
receiving, through the prism coupler 16, the laser beam which is
oscillated in single mode by the optical resonator filter 11 and
output from the external output side end face of the SOA 13.
The light receiving elements 19a, 19b, and 19c are arranged at
positions of receiving the light deviated from the resonator path
in the optical resonator filter 11, where the light receiving
element 19a receives the light from the through port 26t, which is
leaked out without entering the ring resonator 21 from the SOA 13,
the light receiving element 19b receives the light from the through
port 24t, which is leaked out without entering the ring resonator
22 from the SOA 13, and the light receiving element 19c receives
the light from the through port 25t, which is leaked out without
entering the ring resonator 23 from the SOA 13.
The light receiving elements 18, 19a, 19b, and 19c are connected to
the control unit 17. The high reflection film 12 is formed by vapor
depositing or laminating a dielectric multi-layer film on the side
surface of the PLC substrate corresponding to one end of the
reflection side waveguide 27.
FIG. 3 is a block diagram showing a configuration of the control
unit 17 in the exemplary embodiment.
As shown in FIG. 3, the control unit 17 includes an external
interface 31 connected to an external device to retrieve
information indicating the desired wavelength channel value of the
user from the outside; a power supply 32 for supplying powers to
the SOA 13 and the TO phase shifters 14 and 15; a memory 33 for
storing input power values to the TO phase shifters 14 and 15 to
set the resonance wavelength in the multiple-optical resonator 20
to the wavelength channel defined as the ITU-Grid for each
wavelength channel; and a main control unit 34 for controlling the
operation of the control unit 17.
The main control unit 34 has a function of specifying the power
value to be input to the TO phase shifters 14 and 15 from the
information stored in the memory 33 based on the value of the set
wavelength channel input through the external interface 31, and
instructing the power supply 32 to supply powers to the TO phase
shifters 14 and 15; and a function of instructing the power supply
32 to supply power to the SOA 13 and causing the SOA 13 to supply
light towards the optical resonator filter 11 side.
The main control unit 34 further has a function of acquiring the
value of each output light levels detected in the light receiving
elements 18, 19a, 19b, and 19c, adjusting the input power amount to
the phase control region 13A and the TO phase shifters 14 and 15
based on each output light level, and performing a control to
oscillate the laser beam of stable wavelength.
When the light receiving levels of the light receiving elements
19a, 19b, and 19c become large, this means that great amount of
light is leaking from the multiple-optical resonator 20, and the
level of the laser oscillation light is decreasing. On the other
hand, the light receiving levels of the light receiving elements
19a, 19b, and 19c decrease under a condition in which a stable
laser oscillation light output is obtained and the light receiving
level of the light receiving element 18 becomes large. Thus, since
the light receiving characteristic in the light receiving element
18 and the light receiving characteristic in the light receiving
elements 19a, 19b, and 19c are opposite, the laser beam of stable
wavelength can be oscillated by adjusting the light path lengths of
the ring resonators 22 and 23 so as to minimize the light receiving
levels of the light receiving elements 19a, 19b, and 19c and
maximize the light receiving level of the light receiving element
18.
Unless the phase of the light circling in the optical resonator
filter 11 coincides at the side surface of the optical resonator
filter 11 of the SOA 13 and the high reflection film 12, the
insertion loss of the filter appears to increase. Accordingly, the
phase current to be supplied to the phase control region 13A in the
SOA 13 needs to be adjusted to alleviate the insertion loss of the
filter. Thus, in the tunable light source apparatus 10 of the
exemplary embodiment, the light path length of the entire optical
resonator filter 11 also needs to be adjusted with the light path
lengths of the ring resonators 22 and 23.
The main control unit 34 has a function of instructing the power
supply 32 to individually control the input power amount to the TO
phase shifters 14 and 15 until the SOA phase current value at which
the light output detected in the light receiving elements 19a, 19b
and 19c becomes a minimum value and the SOA phase current value at
which the light output detected in the light receiving element 18
becomes a maximum value are matched. Specifically, the input powers
to the TO phase shifters 14 and 15 is fixed firstly, and input
power to the phase control region 13A is adjusted so that the light
receiving level of the light receiving element 18 exhibits a
maximum value. Thereafter, the input powers to the TO phase
shifters 14 and 15 is adjusted until the light receiving levels of
the light receiving elements 19a, 19b, and 19c exhibits a minimum
value, and the input power to the phase control region 13A is
adjusted so that the light receiving level of the light receiving
element 18 exhibits a maximum value in the TO phase condition at
this stage. Such processes are repeated to detect the optimum
points of the input power amount to the phase control region 13A
and the TO phase shifters 14 and 15.
FIG. 4A is a graph showing a state in which the SOA phase current
at which the light output levels from the through ports 26t, 24t,
and 25t become a minimum value and the SOA phase current value at
which the light output level from the input/output port 26i becomes
a maximum value are matched. FIG. 4B shows a state in which the SOA
phase currents are not matched. The main control unit 34 repeats
the above processes until the light receiving characteristics in
the light receiving elements 18, 19a, 19b, and 19c become the
characteristics shown in FIG. 4A.
The main control unit 34 has a function of storing the input powers
to the TO phase shifters 14 and 15 with which the SOA phase current
value at which the light output detected in the light receiving
elements 19a, 19b, and 19c becomes a minimum value and the SOA
phase current value at which the light output detected by the light
receiving element 18 becomes a maximum value are matched in the
memory 33 as phase control amount data, and instructing the power
supply 32 to supply the power indicated by the phase control amount
data to the TO phase shifters 14 and 15 in the next phase
control.
The TO phase shifters 14 and 15 are not controlled by using only
one of the light receiving elements 19a, 19b, and 19c serving as
the second light detector because the light output from the through
ports 26t, 24t, and 25t is a leakage light that did not enter the
ring resonators 21, 22, and 23, and thus, with only one of the
light receiving elements 19a, 19b, and 19c, it can be detected only
whether or not one of the ring resonators 21, 22, and 23 is
coinciding with another filter in central frequency.
For instance, at the detected light level of the light receiving
element 19b, it can be recognized only whether or not the central
frequency in the fine tuning ring resonator 22 is coinciding with
the resonance frequency of other ring resonators 21, 23, and
whether or not the central frequencies of the respective other ring
resonators 21, 23 are coinciding cannot be recognized, and for
example in the figure, it can be recognized only whether or not the
fine tuning ring is coinciding with the other filters in central
frequency in the second light detector of the fine tuning ring
resonator.
The phenomenon that occurs will be described with reference to the
drawings in order to more specifically describe the reasons why the
optimum TO condition cannot be detected with only one of the light
receiving elements 19a, 19b, and 19c serving as the second light
detector.
FIGS. 6 to 8 are views showing TO tolerance of the respective light
receiving elements 19c, 19b, 19a, where X axis indicates the input
power to the TO phase shifter 15 corresponding to the coarse tuning
ring resonator 22, and Y axis indicates the input power to the TO
phase shifter 14 corresponding to the fine tuning ring resonator
23. A case where the entire phase is adapted to an optimum point is
shown throughout FIGS. 6 to 8.
FIG. 6 is a view showing a TO tolerance of the light receiving
element 19c for detecting the light from the through port 24t
corresponding to the coarse tuning ring resonator 23, and shows the
detected light level at the light receiving element 19c in a
contour line. As shown in FIG. 6, the minimum condition of the
detected light level is represented with a dotted line A that
strongly reacts to the X axis. Thus, the optimum point of the TO
condition cannot be detected with only the light receiving element
19c.
FIG. 7 is a view showing a TO tolerance of the light receiving
element 19b for detecting the light from the through port 24t
corresponding to the fine tuning ring resonator 22, and shows the
detected light level at the light receiving element 19b in a
contour line. As shown in FIG. 7, the minimum condition of the
detected light level is represented with a dotted line B. Thus, the
optimum point of the TO condition cannot be detected with only the
light receiving element 19b.
FIG. 8 is a view showing a TO tolerance of the light receiving
element 19a for detecting the light from the through port 26t
corresponding to the ring resonator 21, or the reference ring, and
shows the detected light level at the light receiving element 19a
in a contour line. As shown in FIG. 8, the minimum condition of the
detected light level in this case is represented with a dotted line
C, and although an axis is not present, reacts to a case where the
reference ring and other two rings simultaneously shift. A case in
which the rings simultaneously shift is a case where two rings
shift by the same amount, or a case where Tofine and TOcoarse
decrease or increase by the same amount in FIG. 8.
FIG. 9 is obtained when the three optimum lines A, B, C are
overlapped in the wavelength region, and FIG. 10 is obtained when
the lines are overlapped in the TO tolerance of the light receiving
element 18. A triangular region defined by the three optimum lines
A, B, C shown in FIG. 9 is a stable operational condition, where
the oscillation light output maximum condition exists in the
triangular region defined by the three optimum lines A, B, C, and
the light output characteristic exactly matches the detection
result in the first light detection device as shown in FIG. 10.
The three optimum lines do not coincide at one point as in FIGS. 9
and 10 because the effective light filter characteristics deviate
from the ideal filter characteristics since a ring resonator
propagation loss is present and the light output gradually
decreases. The size of the triangular region becomes smaller, and
concentrates at one point, as the propagation loss in the optical
resonator filter 11 becomes smaller. The sizes of the triangular
region of channel of 1587.9 nm and the triangular region of the
channel of 1587.5 nm of FIG. 9 slightly differ due to influence of
channel dependency of the filter loss.
Since the optical resonator filter 11 presents such light filter
characteristics, the control unit 17 individually controls the
current-flow amount of the TO phase shifters 14 and 15 until the
values of the SOA phase currents at the time when the detection
light intensity of the light receiving element 18 is maximum and at
the time when the light detection light intensity of the light
receiving elements 19a, 19b and 19c is minimum are matched, and
controls the phases of the ring resonators 22 and 23 to easily
search for the optimum conditions.
Therefore, only an optimum line can be detected, and an optimum
point cannot be detected, with only one of the light receiving
elements 19a, 19b and 19c. To detect the optimum condition, either
one of the light receiving elements 19a, 19b and 19c and the light
receiving element 18 are required to be used, or, two of the light
receiving elements 19a, 19b and 19c are required to be used. Thus,
the light receiving elements 18, 19a, 19b and 19c are used in the
exemplary embodiment.
The function content of the main control unit 34 can be programmed
to be executed by the computer.
In the wavelength variable light source apparatus 10 of the
exemplary embodiment, the ASE light emitted from the SOA 13 returns
through a path of SOA 13.fwdarw.input/output side waveguide
26.fwdarw.multiple-optical resonator 20.fwdarw.reflection side
waveguide 27.fwdarw.high reflection film 12.fwdarw.reflection side
waveguide 27.fwdarw.multiple-optical resonator
20.fwdarw.input/output side waveguide 26.fwdarw.SOA 13.
Each ring resonator 21, 22, and 23 configuring the multiple-optical
resonator 20 has different FSR, where greater reflection or
transmittance occurs at the wavelength on which the periodic change
of reflection or transmittance generated in the ring resonator 21,
22, and 23 coincide, and thus the returning light reflected by the
high reflection film 12 from the SOA 13 becomes the strongest at
the resonance wavelength of the multiple-optical resonator 20.
In the tunable light source apparatus 10 of such exemplary
embodiment, the light receiving element 18 receives the laser beam
from the input/output port 26i, and the light receiving elements
19a, 19b, and 19c receive the light from the through ports 26t,
24t, and 25t. The control unit 17 controls the input powers to the
TO phase shifters 14 and 15 formed on the ring resonators 22 and 23
based on the light receiving levels. Thus, the oscillation laser
light becomes more stable.
The operation of the tunable light source apparatus 10 of the
exemplary embodiment will now be described. The control method of
the tunable light source apparatus according to the present
invention will be simultaneously described showing each step.
FIG. 5 is a flowchart showing the operation of the control unit 17
according to the exemplary embodiment.
In the tunable light source apparatus 10, firstly, the power
corresponding to the wavelength channel set by external input is
supplied to the TO phase shifters 14 and 15 and the light path
lengths of the ring resonators 22 and 23 are fixed by the control
unit 17. Then, the ASE light is output from the SOA 13 (light
supply step). The ASE light enters the input/output side waveguide
26 from the input/output port, and is propagated through the
multiple-optical resonator 20 and reflected by the high reflection
film 12, and again propagated through the multiple-optical
resonator 20, and exit from the end face of the SOA 13. The
multiple-optical resonator 20 thereby serves as a laser resonator,
and the laser beam is oscillated from the tunable light source
apparatus 10.
In this case, the light output from the input/output port 26i and
the light output from the through ports 26t, 24t, 25t are detected
by the light receiving elements 18, 19a, 19b, and 19c (light
detection step). The power is supplied to the TO phase shifters 14
and 15 based on the light received amount detected by the light
receiving elements 18, 19a, 19b, and 19c, thereby tuning control of
the phases of the ring resonators 22 and 23 is performed (tuning
control step).
Specifically, the control unit 17 fixes the input powers to the TO
phase shifters 14 and 15 first (FIG. 5: step S31), acquires the
output light levels detected by the light receiving elements 18,
19a, 19b, and 19c, and adjusts the power input to the phase control
region 13A so that the light receiving level of the light receiving
element 18 takes a maximum value (FIG. 5: step S32). Thereafter,
the control unit 17 adjusts the powers input to the TO phase
shifters 14 and 15 until the light receiving levels of the light
receiving elements 19a, 19b, and 19c take a minimum value (FIG. 5:
step S33), and adjusts the power input to the phase control region
13A so that the light receiving level of the light receiving
element 18 takes a maximum value in the TO phase condition of this
stage (FIG. 5: step S34).
As a result of repeating the above, if the SOA phase current value
at which the levels of the light output from the through ports 26t,
24t and 25t detected in the light receiving elements 19a, 19b and
19c become a minimum value and the SOA phase current value at which
the level of the light output from the input/output port 26i
detected in the light receiving element 18 becomes a maximum value
are matched, then the central frequencies of the ring resonators
21, 22, and 23 are matched, and thus the control unit 17 fixes the
powers input to the TO phase shifters 14 and 15 to maintain the
relevant state (FIG. 5: step S36).
If the SOA phase current value at which the light output levels
detected in the light receiving elements 19a, 19b and 19c become a
minimum value and the SOA phase current value at which the light
output level detected in the light receiving element 18 becomes a
maximum value are not matched, the control unit 17 changes the
powers input to the TO phase shifters 14 and 15, and tuning
controls the light path lengths of the ring resonators 22 and 23
until the SOA phase current values are matched.
The graph showing one example of the SOA phase characteristic of
each output light level is shown in FIGS. 4A and 4B. FIG. 4A is a
graph showing a state in which the SOA phase current at which the
level of the light output from the through port becomes a minimum
value and the SOA phase current value at which the level of the
light output from the input/output port becomes a maximum value are
matched. FIG. 4B shows a state in which the SOA phase currents are
not matched. In the exemplary embodiment, the light path lengths of
the ring resonators 22 and 23 are tunably controlled until the
measurement result of FIG. 4A is obtained.
In the exemplary embodiment, the light receiving elements 19a, 19b,
and 19c are mounted at positions of detecting in the middle the
leakage of the light in the mid-course from the SOA to the light
reflection film 12, as shown in FIG. 1, but is not limited thereto,
and may be mounted at positions corresponding to the through ports
for outputting the leakage light of the light returning from the
light reflection film 12, as shown in FIG. 11.
As described above, in the tunable light source apparatus 10 of the
exemplary embodiment, the light receiving elements 19a, 19b and 19c
for detecting the light from the through ports 26t, 24t and 25t and
the light receiving element 18 for detecting the light from the
output port 26i are attached, and the light path lengths of the
ring resonators 22 and 23 are tunably controlled in an aim of
obtaining a state in which the levels of the light output from the
through ports 26t, 24t and 25t are minimum and, at the same time,
the level of the light output from the output port 26i is a
maximum. Therefore, the central frequencies of the three ring
resonators 21, 22, and 23 exactly match the set frequency by
obtaining a state in which the resonance loss is a minimum and, at
the same time, the oscillation light intensity is a maximum,
whereby the laser beam of stable wavelength can be oscillated. The
frequency of the laser beam can be locked within a range of about 1
GHz from the set frequency.
The frequency lock control of the exit light in the tunable light
source apparatus 10 of the exemplary embodiment is executed based
only on the intensity of the light output from the through ports
26t, 24t, and 25t and the output port 26i, and the wavelength
component of the output light does not need to be detected, and
thus a rapid control can be realized.
As an exemplary advantage according to the invention, the present
invention provides a tunable light source apparatus including a
multiple-optical resonator, where a control to exactly coincide the
central frequency of each optical resonator in the multiple-optical
resonator with the set frequency is executed based only on the
intensities of the output lights from the through port and the
output port, and the frequency of the output laser beam can be
locked within a range of about 1 GHz from the set frequency without
detecting the wavelength component of the output light.
In the tunable light source apparatus, the control unit described
above may have a function of controlling the tunable device so that
an intensity of the light detected by the first light detection
device takes a minimum value and an intensity of the light detected
by the second light detection device takes a maximum value.
Therefore, a stable laser beam with small resonance loss is
obtained by adjusting the resonance wavelength of the
multiple-optical resonator in an aim of obtaining a state in which
the oscillation light intensity becomes a maximum and, at the same
time, the intensity of the light from the through port becomes a
minimum.
In the tunable light source apparatus, the second light detection
device may be arranged in plurals with respect to one through port
in correspondence to each optical resonator. The resonance
wavelength of the multiple-optical resonator then can be more
accurately adjusted.
In the tunable light source apparatus, the tunable device may be
configured to individually change the resonance wavelength of each
resonator in the multiple-optical resonator. The resonance
wavelength of the multiple-optical resonator can be adjusted by
changing the resonance wavelength of each resonator of the
multiple-optical resonator.
In the tunable light source apparatus, the resonator in the
multiple-optical resonator may be a ring resonator. Since the ring
resonator is a passive optical component, current injection to the
semiconductor laser and mechanically movable member are not used,
and thus reliable characteristics are obtained.
In the tunable light source apparatus, the tunable device may be a
film shaped heater for changing the light path length of one of the
optical resonators. With this, the resonance wavelength of the
resonator can be changed based on the temperature characteristic of
the waveguide forming the resonator.
In the tunable light source apparatus, the light supply device may
be a semiconductor optical amplifier. With this, the light supply
device can be very miniaturized.
In the tunable light source apparatus, a planar lightwave circuit
may be formed on the same substrate. With this, a precise waveguide
pattern can be formed.
In the above, control method, the control unit may control the
operation of the tunable device so that an intensity of the output
light output from the output port takes a maximum value and, at the
same time, an intensity of the light from the through port takes a
minimum value, in the tuning step.
According to the control method, a stable laser beam with small
resonance loss can be output from the tuning light source apparatus
by adjusting the resonance wavelength of the multiple-optical
resonator in an aim of obtaining a state in which the oscillation
light output becomes a maximum and, at the same time, the light
output from the through port becomes a minimum in the tuning light
source apparatus including the multiple-optical resonator.
The exemplary embodiment of the invention may be built as a tuning
light source apparatus control program. The wavelength variable
light source apparatus control program according to the present
invention is built to cause a computer for controlling the
operation of a tuning light source apparatus including an optical
resonator filter with a multiple-optical resonator in which a
plurality of optical resonators is connected, a light supply device
for supplying light into the filter from the input port of the
optical resonator filter, and a tuning device for changing the
resonance wavelength of the multiple-optical resonator, to execute
a light supply process for instructing the light supply device to
supply light into the optical resonator filter, an output light
intensity input process for inputting, from a light detection
device arranged in advance, intensity data of the light to be
output to the outside from an output port of the optical resonator
filter, a through light intensity input process for inputting, from
a light detection device arranged in advance, the intensity data of
the light deviated from the resonator path in the filter to be
output from a through port of the optical resonator filter, and a
tuning process of controlling the operation of the tuning device
based on each intensity data input in the through light intensity
input process and the output light intensity input process.
In the tuning light source apparatus control program, the content
may be specified to control the operation of the tuning device so
that an intensity of the output light output to the outside from
the output port takes a maximum value and at the same time an
intensity of the light output from the through port takes a minimum
value in the tuning process.
According to such program, the stable laser beam with small
resonance loss can be output from the tuning light source apparatus
by changing the resonance wavelength of the multiple-optical
resonator in an aim of obtaining a state in which the oscillation
light level becomes a maximum and, at the same time, the level of
the light output from the through port becomes a minimum with
respect to the tuning light source apparatus including the
multiple-optical resonator.
It should be apparent to those skilled in the art that the present
invention may be embodied in many other specific forms without
departing from the spirit or scope of the invention. Therefore, the
present invention is not to be limited to the details given herein,
but may be modified within the scope and equivalence of the
appended claims.
* * * * *